While reliable sealing methods utilizing mechanical braided packing and the like have been devised in the past, invariably with mechanical braided packing, there is leakage from within the stuffing box between the packing and shaft to the exterior atmosphere through the annulus formed between the packing gland and pump shaft, despite the best effort of mechanics to minimize such leakage. This leakage has become a severe environmental problem.
Presently, environmental legislation mandates both water conservation and provides standards relating to the amount of material which can be allowed to escape into the environment from a plant, and in particular, from the stuffing boxes used to seal pumps. These requirements vary from location to location and are dependent upon the kind of quality of fluid effluent. The reason for the variation in the standard is that in general, there are so-called cluster rules, which define the allowable water usage and effluent permitted to escape from multiple installations. While it may be that certain power plants in the area may comply with the environmental rules and regulations, other plants in the same area may not comply. It was for this reason that cluster rules were adopted so that a company could comply with the regulations by satisfying an overall requirement for all of the sites within given area.
One way to comply with environmental requirements is to provide packing in which no leakage occurs. This however, has proved elusive, with attempts at eliminating leakage by tightening down of the gland, resulting in undue shaft wear and pump failure. Moreover, even with the use of newer packings, leakage still exists due to pump surges. When this happens, the fluid which is being pumped leaks out from the annulus between the pump shaft and the stuffing box packing gland. Sometimes as little as a 1 psi surge can cause an uncontrollable leak of product from the stuffing box, regardless of what packing is being used. This is known as a "blow out" within the industry.
Recent developments in packing materials involve the use of exfoliated graphite, which results in very little leakage for a reliability of 95%. However, pump surges still result in leakage, and pose a possible non-compliance problem.
It will be appreciated that exfoliated graphite packings have been used in the past in an effort to comply with environmental standards by running dry to eliminate seal water consumption. Thus, it has become desirable to eliminate seal water wherever possible. In place of the clean seal water, the fluid under the packing becomes the fluid pumped. In these cases, oftentimes, the pumps operate "dry". Increasing gland pressure on the packing is used to prevent any leakage of product from the stuffing box. However, this results in shortened packing and sleeve life, increases power consumption, and requires added maintenance costs.
More particularly, recent development of packings made from certain yarns has lead to methods of increased reliability of "dry" running with zero product leakage. These methods are not perfect however, and have a success rate that varies between 50-95% depending on the kind of packing yarn used, and upon the braided construction of the packing. Exfoliated graphite yarns have been produced which are very malleable, provide increased heat transfer and are suitable for use in a range of applications. As will be seen hereinafter, when used in combination with graphite filaments, these yarns provide a vehicle to permit hydrodynamic sealing.
In applications where exfoliated graphite packings will not provide adequate seals, clean seal water is still used for lubrication and to prevent the dirtier pumped fluids from leaking out of the stuffing box. However, this clean sealing water also leaks from the stuffing box.
There is therefore a necessity for providing a reliable system in which leakage to the outside world from the stuffing boxes can be virtually eliminated.
In the past, there have been so called hydrodynamic oil seals which through the rotation of the rotary apparatus forces the oil back towards its source thus to eliminate leakage. Such devices were originally proposed by Chicago Rawhide Corporation of Chicago, Ill. More recently, hydrodynamic face seal technology has emerged in which gas is used as the sealing fluid, with the gas being maintained within the original chamber through a face seal having a number of channels and apertures which maintain the gas within the chamber. However, with respect to pumps and the utilization of mechanical braided packing, no such hydrodynamic sealing devices have been utilized.